1. Field of the Invention
This invention relates to compositions for controlled dispensing of a dry electrolyte composition into a solution of the electrolyte composition. More particularly, this invention relates to a device for dispensing a dry electrolyte composition by osmotic pumping into an aqueous electrolyte solution for an electrochemical apparatus or process.
2. Description of the Prior Art
Prior to the present invention electrolytes for battery or fuel cell systems were delivered from dry electrolyte complexes stored in shaped solids including powder or pellets. Unfortunately, the present forms for delivering electrolyte render it difficult to control the dissolution rate of the dry electrolyte composition into the surrounding aqueous electrolyte composition during operation of the electrochemical apparatus or process. While some degree of electrolyte rate delivery can be attained by controlling the geometric size and shape of the electrolyte composition, the degree of control is too imprecise to properly control the aqueous electrolyte composition over the life of the solid electrolyte composition when its size and shape are constantly changing. One specific example of this problem is the dissolution rate sensitivity of shaped solids to the flow rate of the electrolyte. The release rate for an osmotic pump coated electrolyte will be independent of the flow rate of aqueous electrolyte around it.
Devices for delivering pharmaceutical compositions by diffusion through a permeable polymer coating for the pharmaceutical composition are well known in the art. Included in these delivery systems are those which function by means of osmotic pumping. In such a system, the pharmaceutical composition is enclosed by a continuous semi-permeable coating such as a capsule or a film having a halo of predetermined size therethrough. The device containing the pharmaceutical composition is placed in an appropriate aqueous composition in a place such as the stomach or cul-de-sac of the eye where it imbibes water through the semipermeable membrane. At least a portion of the pharmaceutical is dissolved which causes an increase in osmotic pressure which causes a solution of the pharmaceutical being continuously pumped from the enclosed capsule or the film through at least one hole in the capsule or the film. Similar devices are available as a means for delivering pesticides.
In these systems, the environment surrounding the delivery system is essentially free of the solute being delivered and, is in the case of a pharmaceutical or pesticide is consumed or removed from the immediate environment of the device. Thus, in such systems, the delivered composition is released into a theoretically infinite dilute environment. The release rate is almost always a constant and is dependent on temperature, pump pore size, and permeability of the pump coating. Only at startup and at the end of the device's useful life does the release rate deviate from a constant value.
Representative delivery systems are disclosed, for example, in U.S. Pat. Nos. 4,562,794; 4,670,250; 4,687,660 and 4,769,027.
Prior to the present invention, delivery systems based upon internal osmotic pressure within discrete small vessels have not been utilized to maintain the concentration of a composition in the environment outside a delivery system. Such a system would be useful, for example, for controlling the composition of electrolyte in an aqueous composition surrounding the delivery system also containing an electrolyte.